Huge temp fluctuations. Solutions?

[albertagirl]

Hi, I'm having no luck at all searching for answers on this.
I am having ridiculous temp fluctuations and a really hard time getting them under control, and I don't know why or how to solve it.

My questions are:
Has anyone else dealt with this, and how did you solve it?
What range of temperature fluctuation on the hot side of the cage is ok, and at what point do I need to worry? To me, 10 to 12 degrees F seems like a lot. What do you think?
Does anyone think the thermostat itself could be the problem? Or does it indeed sound like a heat dissipation issue? Or does it sound like heat tapes going bad?

Pertinent details:
I'm using 2' of 12" heat tape under the 3x2 pvc cages (I think it's 1/2" pvc, but might be 3/8"?), and 1' of 12" tape under the 2x2's. I have a Vivarium Electronics Hobby Stat controlling 4 of the cages, and a Jump Start digital thermostat controlling the other 4.

The issue:
Both of my thermostats are controlled by a probe, which turns the heat tapes on and off to control temperature. The problem I'm having is that for some reason the cages on the Jump Start are continuing to increase in temperature for about another 4 degrees C (approx 7 F) even after the tape is turned off. I can't understand how this could be any different from one thermostat to the next since the heat tape is not only identical, but it's also OFF. The cages are also identical, as are the setups. Same approx depth of substrate, same thermometers, same cork bark hides over top.

Because of the temperature continuing to climb well after the heat has turned off, I have to set the thermostat really low so that it turns off well before the max target temp is reached, which would be fine except that this means that the temps then have to drop really low in order for the thermostat to turn back on again, resulting in a huge temp swing. So the thermostat is now set to 80F. Heat tapes turn on once temps drop below 78F, and then turns off at 82F and keeps climbing to about 89F before starting to drop again.

The only thing I can think of that might be the problem is that the heat is not dissapating properly? I've tried reducing the amount of substrate to allow for faster heat dissipation, with no real improvement. I could try increasing the amount of airspace below the cage (which would suck as it would mean tearing the whole stack down), but it has the same airspace as the others, so again, I'm not sure if that would help.

Thanks!